1. Field of the Invention
The present invention relates to the recovery of phosphate values from phosphate rock. More specifically, the present invention relates to a reagent and method for its use in flotation processes for beneficiating phosphate values from phosphate ores.
2. Description of the Prior Art
In the past, ores have been ground before froth flotation treatment in order to liberate one or more mineral species from a second mineral species in order to selectively float one species from the other. In addition, grinding of the minerals created new surfaces which were more responsive to flotation treatment. However, in the flotation of Florida phosphate ore, grinding of the mined ore has not generally been used.
In the processing of phosphate ore (also called the "matrix"), the ore is pumped from the fields in the form of a slurry and is first fed to a washing apparatus. In the washer, the slurry is pumped over a series of screens interspersed with log washers which act to break up clay balls and other large pieces in the matrix. Usually, there are three separate streams exiting the washer as shown in the flow diagram of the prior art process illustrated in FIG. 1. One is a phosphate pebble product stream typically having a BPL of about 65% and a particle size within the range of about 1 millimeter to about 3/4 inch (+16 mesh). A second stream containing both phosphate values and insoluble siliceous minerals or gangue (i.e., sand) has an intermediate particle size range between about 0.1 and 1 millimeter (-16 mesh to +150 mesh). The third stream comprises clay slimes having a particle size below about 0.1 millimeter (-150 mesh). The slimes are typically discarded into a slime pond where the clay eventually settles. Of these three product streams, only the second is subjected to further processing.
In a conventional operation, the second stream is fed to a sizing apparatus which typically divides the phosphate and siliceous mineral containing fraction into three distinct particle size ranges. The finest of these three streams has a particle size range of about 0.1-0.4 millimeters (-35 mesh to +150 mesh). This stream is subjected to a fine flotation step using for example well-known anionic conditioning reagents wherein fine rough siliceous tailings are removed (and wasted) and a fine rough phosphate concentrate is collected in the froth.
The intermediate particle size stream coming from the sizing apparatus has a particle size range of about 0.4-0.7 millimeters (-24 mesh to +35 mesh) and is fed to a coarse flotation unit which also uses conventional anionic conditioning reagents. In the coarse rough flotation a coarse rough tailing is removed and may be wasted (or recycled after further sizing) and a coarse rough phosphate concentrate is collected in the froth and can be combined with the fine rough concentrate mentioned earlier.
The combined streams of the fine rough concentrate and the coarse rough concentrate then generally are sent to an acid (typically H.sub.2 SO.sub.4) scrubbing unit to remove the fatty acid and fuel oil reagents. The acid scrubbed slurry then is washed with water and sent to a cationic (amine) flotation unit. Before flotation, the feed slurry is conditioned with a flotation reagent typically comprising a mixture of an amine and kerosene. The particle size of the material going into the amine flotation unit is very fine, with 82-90% being less than about 0.4 millimeters (-35 mesh). It is well known that amine flotation of quartz from phosphate is ineffective with coarser particle sizes (i.e., +35 mesh). See Cooke (1949), Mining Transactions, 184:306-309 and de Bruyn et al (1956), Mining Engineering, April, pp. 415-419. In the amine flotation, siliceous mineral impurities are removed in the froth and a phosphate concentrate, typically having a BPL value in the range of about 71-72% is collected in the cell underflow as product.
The third stream exiting from the sizing apparatus comprises particles having a very coarse particle size in the range of about 0.7-1.0 millimeters (-16 mesh to +24 mesh). The stream is beneficiated by a combination of chemical conditioning and mechanical separation techniques using a skin flotation device such as a spiral separator, a belt separator, a concentrating table or the like. As noted above, the conventional "double float" process using sequential anionic and cationic conditioning steps cannot be used to beneficiate this fraction because the cationic reagents are not effective for floating siliceous impurities of such large particle sizes. Thus, to beneficiate this fraction the art are relied on mechanical techniques to enhance the separation obtained using anionic reagents.
Normally this stream is chemically conditioned at a high solids concentration with a conventional anionic conditioning reagent such as a mixture of a fatty acid reagent, such as tall oil, and a fuel oil extender. The conditioning reagent may also include ammonia or caustic for pH control. The stream then is fed to the skin flotation device.
Probably the best skin flotation device is a spiral separation unit such as available from Jensco, Inc., Eaton Park, Fla. These devices comprise a series of downwardly sloping spiral troughs having a number of side exit ports in the trough along the inner edge thereof. The heavier siliceous materials tend toward the inside of the spiral trough while the lighter reagentized phosphate materials tend toward the outside. The inside exit ports are positioned to accomplish separation of the heavier siliceous materials. The spiral tails containing the siliceous minerals are then sent to a scavenger flotation cell wherein residual phosphate values are foamed to the top, while the heavier siliceous minerals are wasted from the bottom of the cell.
The spiral concentrate streams and the scavenger flotation cell streams generally then are combined to produce another stream typically having a BPL value of about 68% and an insoluble fraction of about 8-12%.
The spiral units and the other skin flotation devices used to separate siliceous mineral gangue from phosphate values in the very coarse (greater than about 0.7 millimeters (+24 mesh)) particle size range generally are troublesome pieces of equipment. Such devices have limited capacity per unit area. In the case of the spiral separator, rather small streams must be used in the spiral troughs and hence, for reasonable production numerous spiral units must be used. Since the streams are typically dirty, the units quickly become fouled and must frequently be shut down for cleaning.
In addition, while such equipment is almost universally used for separating siliceous minerals from phosphate values in the coarse particle size ranges, these devices are recognized to be inefficient separators. Thus, there has been a long felt need in the art to replace such skin flotation apparatus with equipment of much simpler, smaller and trouble-free design and operation. While flotation cells comprise a logical piece of equipment to replace the skin flotation apparatus, as noted above conventional flotation techniques have not been effective for floating the coarser size siliceous materials. For instance, the use of only a flotation cell without the prior use of a spiral unit would result in the loss of significant phosphate values.
U.S. Pat. No. 2,904,177 to Michal discloses a process for removing silicates, by flotation, from ilmenite ore (FeTiO.sub.3) in order to recover titanium values. The disclosed process comprises grinding the ore to less than 60 mesh (-60 mesh) particle size and preparing an aqueous pulp therewith. Hydrofluoric acid is added as a regulator to acidify the pulp to a pH in the range of 3.0-6.0. Starch is added to depress the titanium. A cationic amine flotation agent, such as a quaternary ammonium salt of the higher aliphatic series, is added. Optionally, a frothing agent, such as pine oil, also may be added. The mixture then is subjected to froth flotation whereby siliceous impurities are separated by flotation from the titanium values.
U.S. Pat. No. 2,970,688 to Uhland discloses a typical two-step flotation process. The phosphate ore is first ground, sized, deslimed, and placed in an aqueous pulp. An anionic flotation agent having the ability to carry phosphate and heavy mineral values to the froth is added to the less than 35 mesh particle size fraction of the ore. The froth is recovered, washed and then reagentized with a cationic flotation agent having the ability to carry silica and heavy mineral values to the froth. The reagentized material then is subjected to a second flotation step. The cationic flotation agents disclosed include high molecular weight aliphatic quaternary ammonium bases and their water soluble salts.
U.S. Pat. No. 2,914,173 to LeBaron discloses a similar two-step flotation process for beneficiating the less than 35 mesh particle size fraction of a phosphate ore. In the second flotation step, a cationic flotation agent again is used. High molecular weight aliphatic quaternary ammonium bases and their water soluble salts are disclosed as possible flotation agents. There also is disclosed the addition of a cationic flotation agent in combination with kerosene.
As evident, the use of quaternary ammonium compounds for floating siliceous mineral impurities from a desired ore is known. As is the case with cationic collectors generally, however, the prior art has limited their use to flotation of small sized particles.